Proton exchange membrane fuel cells (PEMFC)s are promising clean power sources for vehicular transportation, residential and institutional, and also for computers and mobile communication equipment1. As one of the key components of the membrane electrode assembly (MEA), proton exchange membranes (PEM)s carry catalyst, provide ionic pathways for protons and prevent crossover of gases or fuel. Perfluorosulfonic acid PEMs, such as Dupont's Nafion® membrane, are typically used as the polymer electrolytes in PEMFCs because of their excellent chemical and mechanical stabilities as well as high proton conductivity. However, their disadvantages of high cost, low operation temperatures and high fuel permeability are stimulating an intensive search for alternative materials.
Amongst recently developed polymer electrolyte membranes, sulfonated poly(arylene ether ketone)s (SPAEK)s and sulfonated poly(arylene ether sulfone)s (SPAES)s are promising2-21. For example, the conductivity of sulfonated Victrex™ PEEK with a SC of 0.65 reaches 0.04 S/cm−1 at 100° C./100% RH. In 2002, Wang and McGrath9 reported the synthesis of biphenyl-based sulfonated poly(arylene ether sulfone)s by direct polymerization reactions of disodium 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS), 4,4′-dichlorodiphenylsulfone and 4,4′-biphenol. The conductivity values at 30° C. for the 40% SDCDPS copolymer and the 60% SDCDPS copolymer were 0.11 S/cm and 0.17 S/cm respectively. In addition, compared with post-sulfonation reactions, this method allows close control the sulfonate content of polymers and avoids possible crosslinking or other side reactions that could occur under modification conditions. Our group and Xiao et al also reported the preparation and conductivity results of sulfonated poly(phthalazinone ether ketone)s and sulfonated poly(phthalazinone ether sulfone)s by both sulfonation reactions and direct polymerization reactions12-16. Both methods gave sulfonated polymers with conductivities higher than 10−2 S/cm at around SC 1.0.
In sulfonated polymer membrane films, the hydrophobic backbone and the hydrophilic sulfonic acid groups nanophase separate into two domains in the presence of water. The hydrophobic domain provides the polymers with morphological stability and the hydrophilic domain is responsible for transporting protons and water19, 20. Compared with perfluorinated sulfonic acid membranes, sulfonated poly(aryl ether ketone)s are reported20 to have a smaller characteristic separation length and wider distribution with more dead-end channels and a larger internal interface between the hydrophobic and hydrophilic domains as measured by small angle X-ray scattering (SAXS)20. However, if short pendant side chains between the polymer main chain and the sulfonic acid groups exist in the polymer structure, the nano-phase separation of hydrophilic and hydrophobic domains may be improved and the amount of dead-end pockets decrease7, 22. Rikukawa and his coworkers7 prepared sulfonated PEEK (SPEEK) and sulfonated poly(4-phenoxybenzoyl-1,4-phenylene, Poly-X 2000) (SPPBP) by post-sulfonation reactions of corresponding parent polymers. They found that SPPBP, which has pendant side chains between polymer main chain and sulfonic acid groups, showed higher and more stable proton conductivity than SPEEK. Jannasch and co-workers devised a new route22 to increase the distance of sulfonic acid groups from the polysulfone main chain via lithiation of polysulfone23 followed by anionic reaction with sulfobenzoic acid cyclic anhydride. Miyatake and Hay24 synthesized copolymers containing sulfonated tetraphenylene and fluorinated alkane moieties with sulfonic acid groups attached onto pendant phenyl groups by the post-sulfonation reaction of corresponding polymers.
Sodium 6,7-dihydroxy-2-naphthalenesulfonate (DHNS) is a commercially available and inexpensive naphthalenic diol containing a sulfonic acid side group, which is widely used in dye chemistries.